Sur la limite supérieure de l'étage Maastrichtien dans la région-type

Résumé En

In the type-region of the Maastrichtian of Dumont for a long time strata have been recorded which differ considerably in faune from the underlying Maastrichtian tuffaceous chalk. Recently they were again discovered in the neighbourhood of Geul-hem (6 kms NE of Maastricht) and, on account of the microforami-nifera they yielded, ascribed to the Paleocene by Hofker. The definition of the age of these layers offers an important strati-graphical problem : the fixation of the upper limit of the Maastrichtian and its relations with the Secondary-Tertiary boundary, which problem occupies many micropaleontologists and stratigraphers in the world at present. In 4 localities layers were observed differing from the underlying tuffaceous chalk in lithological aspect by the presence of a more or less large quantity of glauconite. They are separated from the latter by a typical hard ground or by a series of coarse strata rich in fossils conspicuously reworked from the Maastrichtian. The hard ground contains many Ammonites, Baculites and other typical cretaceous Molluscs and the underlying tufïaceous chalk, characterized by abundant Hemiaster prunella, yielded several Belem-nella Casimir ovensis, which is generally considered as the last Belem-nite of the Maastrichtian. The overlying glauconitic tuffaceous chalk did not yield any Ammo¬ nite or Belemnite, but a macrofauna composed of species which do generally not occur in the Maastrichtian. Crania brattenburgica, a characteristic Brachiopod of the Danian of Denmark, is represented here by the subspecies Cr. brattenburgica geulhemensis. Examination of the planktonic Foraminifera from samples taken at several levels and spots in the various sites showed a marked change in microfaune above the hard ground or its equivalent. In the tuffa-ceous chalk planktonic Foraminifera, though relatively rare, were found forming an association of typical cretaceous species, with the exception, however, of a representative of the group Girt, compressa, species until now known from Post-Maastrichtian deposits only. In the glauconitic tuffaceous chalk the planktonic fraction consists of an entirely different association, with dominating species Gl. daub-jergensis, which species characterizes Danian strata all over the world. The author concludes that the upper limit of the Maastrichtian in the type-region is characterized by a typical hard ground or its lateral equivalents. The overling strata are, especially on account of the planktonic Foraminifera they contain, ascribed to the Danian.